RFID systems have been used to manage and track inventory in consumer retail stores and other businesses having large inventories. In a typical asset-tracking application, an RFID system includes at least one RFID reader and at least one RFID tag, and the RFID tag may be placed upon the asset to be tracked. An RFID tag has stored data, such as stored in an RFID tag memory, that identifies the tag, and the RFID tag typically transmits the stored data, such as stored in an RFID tag memory in response to a time varying Radio Frequency (RF) wave transmitted by an RFID reader. RFID tags generally fall into two categories: semi-passive or active tags that include an on-board power source (e.g., a battery); and passive tags that draw power from the RFID reader via the radio frequency carrier wave transmitted from the RFID reader.
In a passive RFID tag, the time varying radio frequency carrier wave powers the passive RFID tag by generating an AC voltage across the antenna of the passive RFID tag. The AC voltage is typically rectified to a DC voltage, and the DC voltage builds until the DC voltage reaches a minimum operating DC voltage to activate the passive RFID tag. Once activated, the RFID tag can transmit the stored data such as by modulated backscattering of the carrier wave received from the RFID reader. The RFID tag backscatters by changing the amplitude and/or phase of the RF carrier wave from the RFID reader and modulates the RF carrier wave by altering the load impedance of the RFID tag antenna.
RFID systems typically utilize frequencies that are within one of several frequency ranges including a low frequency range (e.g., from about 30 kHz to about 300 kHz), a high frequency range (e.g., from about 3 MHz to about 30 MHz), an ultra high frequency range (e.g., from about 300 MHz to about 3 GHz), and a microwave range (e.g., greater than about 3 GHz). Although these frequency ranges are commonly used, other frequency ranges may also be used. The assigned frequency range is often channelized (e.g., split into multiple channels) to allow simultaneous operation of multiple RFID readers.
Despite having frequency diversity (e.g., multiple RF frequency bands of operation), variations in RFID tag orientation and RF multipath propagation may decrease the read probability of the RFID tag. Some factors that typically decrease the read probability of an RFID tag include the orientation of the RFID tag with reference to the RFID reader (e.g., an RFID reader positioned off the end of an RFID tag antenna), cross polarization (e.g., a vertically oriented RFID tag with a horizontally polarized RFID reader signal), location of the RFID tag behind an electromagnetically absorptive or reflective product, location of the RFID tag within a signal null of a reflective environment, and location of the RFID tag within a cross-polarized null region when a circular polarized RFID reader signal undergoes multipath reflection (e.g., an RFID reader transmitted clockwise signal and a reflected counterclockwise signal may combine to create a primarily linear polarized signal that is cross-polarized to the RFID tag).
Common surfaces associated with storing or displaying inventory, such as flooring or shelving, may reflect the radio frequency carrier waves transmitted from the RFID reader to produce the multipath null. Each carrier wave frequency band has a substantially limited frequency range of operation. For example, a 915 MHz frequency band has a range of about 2.8% frequency difference (e.g., from about 902 MHz to about 928 MHz) for operation. This swept-frequency characteristic of the RFID reader causes the multipath null to move but to a slight degree. The read probability may further decrease as the distance decreases between the RFID tag and the reflecting surface. For example, the close proximity between the reflecting surface and the RFID tag may create partial shielding of the RFID reader transmitted signal and further decrease any movement of the multipath null. In the latter case, the multipath null is not likely to move far enough to bring the tag reception signal above the threshold for RFID tag response.
Accordingly, it is desirable to provide an RFID tag and reader system that minimizes reading interference resulting from variations in RFID tag orientation and RF multipath propagation. In addition, it is desirable to provide an RFID tag and reader system having a mode diversity that operates with a variety of signal reflecting/absorbing tagged assets and in a variety of signal reflecting/absorbing environments. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.